Spain's Vicente García de Mateos and his Danish rival Asbgorn Kragh reportedly let their fists fly after getting into a collision after they crossed the finish line, according to Spanish media Mundodeportivo.
The violent incident was captured on video by the race organization's helicopter, with a policeman and other cyclists having to get involved to break up the fight.
García de Mateos and Kragh had come third and fourth in the final sprint of the 155km stage when they came to blows.
The two racers have now been expelled from the Volta a Portugal, organizers said in a statement.
Spanish cyclist, Alejandro Marque won last year's edition of the top race in Portugal.
Following this success, he was recruited by Spanish team Movistar but lost his contract after it emerged he had tested positive for steroid use during the 2013 Tour of Portugal.